Current demineralization or softening systems often make use of ion exchange resins, e.g. in water purification plants, or softening chemicals, e.g. in laundry washing-machines and dishwashing-machines. The main disadvantage of the resins is that they loose their ion exchange capacity after a period of time and need to be regenerated. This regeneration step involves the use of additional chemicals like acids, bases or salts. These chemicals are harmful for the environment because they can cause salinization. The same holds for softening chemicals used in washing formulations used in laundry washing-machines and dish washing-machines. Salinization is known as the accumulation of soluble mineral salts near the surface of soil, usually caused by the capillary flow of water from saline ground water. Where the rate of surface evaporation is high, irrigation can exacerbate the problem by moistening the soil and causing water to be drawn from deeper levels as water evaporates from the surface. The evaporation of pure water leaves the salts behind, allowing them to accumulate, and they can reach concentrations that are toxic to plants, thus sterilising the land.
Alternative demineralisation or softening systems can be based on thermo-regenerable ion exchange resins. These resins can be regenerated at lower or higher temperatures than the temperature at which they are used without the addition of chemicals.
U.S. Pat. No. 4,152,496, WO 2005/103124 and US 2005/0234141, incorporated by reference herein, disclose a polyampholyte resin (hybrid copolymers) that can be thermally regenerated and which is capable of removing ions from a water stream. In particular, WO 2005/103124 and US 2005/0234141, both incorporated by reference, disclose a polyampholyte resin comprising a macroporous “host” copolymer and a second “guest” polymer which is contained in the pores of the macroporous “host” copolymer. The polyampholyte resin can be prepared in a non-aqueous solvent by crosslinking a polyacrylate with vinyl benzyl chloride in the presence of divinyl benzene as crosslinker followed by conversion of the chloride groups into amino groups. When the polyampholyte resin is contacted with water, the carboxylic and amino groups will ionize and form zwitterions consisting of carboxylate and ammonium groups. Upon contacting the resin in a subsequent step with a salt solution, preferably at a temperature of about 5° to 25° C., the salt cations and salt anion will bind to the ammonium and carboxylate groups, respectively. The salt may be removed at a later stage at higher temperatures, preferably at about 60° to 100° C., to regenerate the original resin. A disadvantage of such thermo-regenerable resins is that temperatures that are necessary for regeneration and that the temperature differences between binding the salt ions and regenerating the thermo-reversible resin are rather high (at least 35° C.) and that therefore these systems are energetically ineffective.
The regeneration of this resin is believed to be based on the principle that at high temperature, salts are more easily washed out of the resin when the positive and negative charged functional groups of the resin are placed close together and therefore lack the need of counter ions.
Another thermo-regenerable polymer system known in the art is disclosed in WO 2005/049679, incorporated by reference. This polymer system comprises a non-ionic polymer comprising anionic terminal end groups and can be used to remove multivalent metal cations from an aqueous system. WO 2006/078163, incorporated by reference, discloses a similar polymer system having cationic groups. These polymeric systems bind ions at temperatures higher than the critical micellization temperature (CMT) and can be regenerated at temperatures lower than the CMT. See also Custers et al., J. Am. Chem. Soc. 127, 1594-1595, 2005 and Lauw et al., Langmuir 22, 10932-10941, 2006, both incorporated by reference. The disadvantage of these systems is that binding occurs at elevated temperatures whereas regeneration occurs at lower temperatures. A reversed system, i.e. regeneration at a temperature higher than the CMT, would, however, be more preferable in certain applications such as laundry washing-machines and dish washing-machines.
Another disadvantage of the polymer systems according to WO 2005/049679 and WO 2006/078163 is that in thermo-reversible binding processes, wherein the polymer systems and a metal cation extracting aqueous phase are separated by a semi-permeable membrane, the Donnan-effect (also known as the Gibbs-Donnan effect) results in an uneven distribution of ions on either side of the membrane, which prevents the extraction from metal ions out of the polymer system to the extracting aqueous phase if the latter has a low ionic strength. As a consequence, when using such polymer systems, extracting aqueous phases which have a high ionic strength must be used in order to ‘pull out’ the cations bound to the polymer systems.
WO 96/06134, incorporated by reference, discloses responsive gels based on N-isopropylacryl amide, acrylic acid and acrylamide.
Alvarez-Lorenzo et al., Langmuir 17, 3616-3622, 2001, incorporated by reference herein, discloses a polyampholyte thermo-sensitive gel comprising carboxylate and ammonium groups.
Balamurugan et al., Langmuir 19, 2545-2549, 2003, incorporated by reference, discloses poly(N-isopropylacrylamide) brushes grafted on mixed self-assembled monolayers of gold by atom transfer radical polymerization.
Zhang et al., Polymer 46, 7695-7700, 2005, incorporated by reference, discloses a semi-interpenetrating network (semi-IPN) composed of a crosslinked copolymer of acrylamide/acrylic acid and linear polyallylammonium chloride.
U.S. Pat. No. 4,202,737, incorporated by reference, discloses a desalination process wherein a thermally regenerable ion exchange resin having weakly acidic free acid groups and weakly basic free base groups is contacted with an aqueous feed solution containing a salt of a strong acid. Hence, thermally regenerable ion exchange resin has only neutral free acid groups and neutral free basic groups. The thermally regenerable ion exchange resin may be a hybrid resin as is for example disclosed in U.S. Pat. No. 3,991,017, incorporated by reference, i.e. that it contains two crosslinked copolymer phases (IPN's). The thermally regenerable ion exchange resin may also be a composite ion-exchange resin as is for example disclosed in U.S. Pat. No. 3,645,922, incorporated by reference, i.e. that it contains composite particles of non-crosslinked ion exchange resins (cationic and anionic) which are dispersed in a homogeneous matrix of a crosslinked, water-insoluble polymer material.
Prior to or during the contacting step with the aqueous feed solution, the weakly basic free base groups are converted into the carbonate form of these weakly basic free base groups, e.g. ammonium hydrogen carbonate groups having the general formula resin-[N(R3)H+][HCO3−]. During the contacting step, the HCO3− anion is exchanged with the anion of the salt of the strong acid thereby forming a salt of the cation of the salt of the strong acid and of the liberated HCO3− anion, said salt of the cation of the salt of the strong acid and of the liberated HCO3− anion reacting with the neutral weakly acidic free acid groups under the formation of carbonic acid (H2CO3), wherein the neutral weakly acidic free acid groups are converted in their salt form. As a consequence, during the whole desalination process the acidic groups and basic groups remain neutral (either in the free form or in the salt form).
U.S. Pat. No. 3,991,017 discloses hybrid copolymers (or IPN's) containing ion exchange functional groups (cationic and/or anionic), wherein the hybrid copolymers contain a crosslinked macroreticular host of a polyunsaturated monomer and a monethylenically unsaturated monomer, wherein said cross-linked macroreticular host is partially filled with a crosslinked gel copolymer phase of a polyunsaturated monomer and a monoethylenically unsaturated monomer. Hence, the hybrid copolymer consists of two cross-linked copolymer phases.
U.S. Pat. No. 3,645,922, incorporated by reference, discloses a composite adsorbent which is capable of being regenerated by elution with water or saline solutions at a temperature higher than the temperature at which adsorption occurs. The composite adsorbent is in the form of composite particles, wherein the composite particles comprise acidic and basic ion exchange resins which are dispersed in a homogeneous matrix of a water-insoluble polymeric material, the latter being a crosslinked polyelectrolyte or a crosslinked copolymer having neutral hydrophilic functional groups.
Mohan and Geckler, React. Funct. Polym. 67, 144-155, 2007, incorporated by reference, discloses polyampholyte hydrogels (interpenetrating networks) composed of positively and negatively charged units. The hydrogels are prepared from poly(N-isopropylacrylamide-co-sodium acrylate) and poly(ethyleneimine). The use of these polyampholyte hydrogels in ion separation processes is not explicitly disclosed.
Obviously, there is a need in the art for an ion exchange system that can be regenerated at moderate temperatures without the addition of chemicals.